This invention relates to systems for the generation of acoustic signals and more particularly to the generation of Steerable Thermoacoustic Signals using thermoacoustic effects.
A submarine is a ship that can operate both on the surface of the water and completely submerged. In order to avoid detection by radar, surface ships and air patrol, a submarine is usually submerged. Modern submarines have the capability of remaining submerged for long periods of time. In fact, a modern submarine can circumnavigate the earth while running submerged. Thus, modern submarines may complete large portions of their missions while being submerged.
Occasionally, while the submarine is submerged, an airborne vehicle may want to communicate with the submarine. Prior art communications systems used buoys. The airborne vehicle dropped a buoy into the water and the submarine either surfaced to receive the message from the buoy (the message was transmitted to the buoy on an RF Frequency) or the airborne vehicle communicated with the buoy on an RF Frequency and the buoy emitted a sound wave which propagated through the water. The submarine would detect the aforementioned sound waves with sound detection systems like sonar. One buoy would produce omnidirectional sound and directional sound was needed for good communications and detection. The prior art produced directional sound by utilizing an array of buoys which were dropped in the water by an airborne vehicle. The buoys were precisely spaced in the water and beamforming equipment was used to properly phase the beam. Some of the disadvantages of the foregoing systems were that: the buoys had to be carried by an aircraft and the buoys would require space aboard the vehicle and add to the weight of the aircraft, which would reduce the amount of other equipment the aircraft would carry and/or reduce the aircraft""s range; the buoys might be detected by a foreign power and disclose the relative location of the submarine; the buoys had a limited range and as the submarine proceeded on its mission the submarine might travel away from the buoy, necessitating the dropping of another array of buoys so that the transmission between the aircraft and the submarine might be continued; the buoys used an active transducer to convert the signals it received from the airborne vehicle into acoustic noise (the acoustic noise levels were high, which is undesirable from a covertness standpoint); the buoys required beamforming equipment; and the buoys were expendable, which meant that the transmission of a message to a submarine was relatively expensive.
Another method utilized by the prior art for the transmission of messages between an airborne vehicle and a submarine employed the use of a very low frequency antenna. The airborne vehicle would extend a low frequency antenna from its fuselage. This method proved to be disadvantageous, since it changed the performance characteristics of the aircraft and made the aircraft less maneuverable. Furthermore, low frequency antennas were not capable of being installed aboard all types of airborne vehicles.
This invention overcomes the disadvantages of the prior art by creating a Steerable Thermoacoustic Array that is completely mobile by having its transmission equipment aboard an airborne or spaceborne platform and its receiving equipment aboard a device that operates underwater, e.g., submarine, torpedo, mine, underwater oil exploration equipment, etc. The laser or particle beam that is used to produce thermoacoustic signals is steered so that a beam will be produced which traces a path along the water at speed Co/sin xcex8 where Cxcex8 is the speed of sound in water and sin xcex8 is the steering angle. Thus, the foregoing system produces directional signals without any expendable components or easily detectable components that float on the surface of the water. Furthermore, since the receiving equipment can function underwater, a submarine would not have to interrupt its mission (surface) to receive a message from an airborne vehicle or satellite.
The apparatus of this invention achieves the above by utilizing the direct conversion of EM or particle kinetic energy into acoustic energy. The foregoing is accomplished by using either a pulsed infrared wavelength laser or particle beam which is fired into the water from an aircraft or satellite. The physical mechanisms producing sound are of two kinds: (1) thermal expansion of the water from heat generated by medium attenuation of a pulse of laser light or impinging particles, or (2) explosive vaporization of a small volume of water when the heat deposited by the laser or particle beam is large enough to raise the local water temperature above boiling threshold. Infrared laser light is usually used because of its high attenuation coefficient in water which causes high thermal densities. The level of sound produced by infrared lasers is sufficient for communications at expected ranges of communication buoys. Infrared lasers may be controlled (modulated) to the extent required for an underwater communications system. Typical data rates are xcx9c1-10 bits per second.
Modulation schemes which may be employed are on-off keying (OOK), pulse duration modulation (PDM), pulse amplitude modulation (PAM), and frequency shift keying (FSK). The foregoing modulation schemes may be used for lasers and particle beams.
When the density of the heat energy deposited by laser beam absorption is less than that required to vaporize a local volume of water (xcx9c2500 joules/cm3) the acoustic pressure at radial distance R and polar angle xcex8 from the beam impact point at the water surface is given by the following expression:       P    ⁢          (              R        ,        T        ,        θ            )        =            k              2        ⁢        π              ⁢                  ∫                  -          ∞                          +          ∞                    ⁢              xe2x80x83            ⁢                        ⅆ          ω                ⁢                  xe2x80x83                ⁢                  M          ⁢                      (            ω            )                          ⁢                  ω          2                ⁢                              exp            ⁡                          [                              -                                  j                  ⁢                                      (                                                                  ω                        ⁢                                                  xe2x80x83                                                ⁢                        t                                            -                                              R                        /                                                  c                          o                                                                                      )                                                              ]                                ·          sin                ⁢                  xe2x80x83                ⁢        θ            
where k=xcex2Io/(4xcfx80RcoCp)
Co=speed of sound
Cp=specific heat of water
Io=laser power output
t=time
xcex2=thermal expansion coefficient of water
Here M(xcfx89) is the Fourier transform of the modulation, and Io the laser power output prior to modulation. The above expression assumes that the useful portion of the acoustic signal is transmitted at a frequency with wavelength smaller than either the beam spot size or absorption depth.
If the modulation is a gaussian pulse       M    ⁢          (      t      )        =                    M        o                              2          ⁢          π          ⁢                      xe2x80x83                    ⁢                      σ            t                                ⁢          exp      ⁡              [                              -                          t              2                                /                      σ            t            2                          ]            
where "sgr"t{tilde over (=)}(one-half of the laser pulse width). The Fourier transform of P(R,xcex8) is proportional to the function F(xcfx89)=xcfx892 exp[xe2x88x92xcfx892"sgr"t2]. The frequency (xcfx89)p when the spectral energy is the acoustic pulse peak is       ω    p    =            1              3              ⁢          σ      t              -        1            
as can be found by setting the derivative of F(xcfx89) equal to zero.
Thus, the duration of the laser pulse (2"sgr"t) controls the spectral Wp. The bandwidth of the signal can be controlled by firing the laser a number of times at a repetition interval less than or equal to the duration of an acoustic pulse produced by a single laser pulse, or by simply lengthening the pulse duration for a single pulse. The pulse amplitude may be controlled and varied by changing the laser power output.
The extremely short 1-10xcexc absorption length for certain infrared light frequencies in water makes an explosive vaporization mode of thermoacoustic generation attractive. Incident light with a fluence of  greater than 3 J/cm2 (ET) at 10xcexc wavelength, for instance, will instantaneously boil the 10 micron layer in which most of the light is absorbed. This rapid vaporization produces an explosive stress or shock wave (with Fourier transform S(xcfx89)) which eventually propagates through the water as a soundwave (with Fourier transform proportional to xcfx89S(xcfx89)). The internal energy (E) contained in the gas that was vaporized is approximately given by the ideal gas state equation:   E  =            3      2        ⁢    PV  
where E is the difference between the laser energy and the energy required to boil the thin layer of water. The initial pressure in the gas bubble would be approximately given by       P    o    =            2      3        ⁢                  (                              E            o                    -                      E            T                          )            V      
where:
Eo=laser pulse energy
ET=Threshold for vaporization
V=Volume of fluid in which absorption of light occurs
V≅Axcex4=(spot area)xc3x97(laser light absorption depth)
Reasonable values for the spot area (A) and absorption lengths are:
A=spot area=1CM2=10xe2x88x924m2 
xcex4=absorption length of fluid=10xe2x88x925m at CO2 laser wavelengths
The determination of allowable communication path length requires a knowledge of the spectral level and distribution of the acoustic energy represented by the source strength given above. The duration of the time domain pulse resulting from explosive vaporization of the water surface layer must be estimated to obtain its spectral distribution. Assume the laser pulse is sufficiently short (xe2x89xa610xe2x88x926 sec.) so that all the laser energy is absorbed before the explosive vaporization has appreciably progressed. The time required to expand the 10xe2x88x929m3 volume of water to 1 ATM gaseous phase is roughly one-half the width of the acoustic pulse produced. The expanded volume of the water is 10xe2x88x926m3 based on the roughly 103 difference in density between liquid water and water vapor at 1 ATM. The vapor bubble expands at roughly Mach two in air (2200 m/sec.) forming a spherical segment of volume xcx9c10xe2x88x926m3. The time for the expansion to take place is
T=4.5xcexcsec.
at Mach two. The center frequency of the wideband pulse thus produced is
fo=(1/(9xcexcsec))=110 Khz.
The spectrum of the thermoacoustic pulse is a roughly 100% bandwidth pulse centered on fo thus with single pulse on-off coding the signal bandwidth is (BW){tilde over (=)}110 KHz.
Taking, for example a 10 joule laser pulse, the peak pressure at the surface is             P      s        =                  2        3            ⁢                        (                                    E              o                        -                          E              T                                )                          A          ⁢                      xe2x80x83                    ⁢          δ                                P      s        =                  2        3            ⁢                        (                      10            -            3                    )                                      (                          10                              -                4                                      )                    ⁢                      (                          10                              -                5                                      )                                or
316 dB relative to 1xcexc Pa (relxcexcPa) Assuming spherical spreading from an initial radius (Ro) of the source, the source strength at a range R is       P    ⁢          (      R      )        =            P      o        ⁢                  (                  R          o                )            R        ⁢          f      ⁢              (        θ        )            
where xcex8 is the horizontal propagation angle, and f(xcex8) is the source directivity (≈sinxcex8). The initial radius can be taken as V ⅓where V=10xe2x88x926m3 so that Ro=10xe2x88x922m. The resulting source strength at 1 meter below the beam impact point (sinxcex8=1) is then
SL=20 log P(1)=293xe2x88x9220 log 104 
or
SL=213 dB re (1xcexcPa)
The standard sonar equation can be used to estimate the excess signal at a distance r meters from the source. In the above example, the spectrum of the acoustic signal is approximately linear with frequency for xcfx89 less than xcfx89p. Thus, the spectrum level (dB//Hz) at 10 KHz (our assumed transmit frequency) is 20 dB below that at 110 KHz. The spectrum level for xcex8=xcfx80/2 rad. at 110 KHz is about 213 dBxe2x88x9210 log (1.1xc3x97105 Hz){tilde over (=)}163 dB//Hz. Therefore, the acoustic spectrum level at 10 KHz≈143 dB//Hz. The sonar equation is inverted to give Figure of Merit (maximum propagation loss) for good communication reliability. This yields (Figure of Merit) FOM=143xe2x88x9245xe2x88x9212=86 dB=source spectrum levelxe2x80x94noise spectrum levelxe2x80x94threshold.
The signal-to-noise ratio required to reliably communicate is assumed to be 12 dB. The range of the signal pulse on-off keyed communication system described above corresponding to an 86 dB FOM is 6 Kyd. Receiving the signal with a directional receiver will increase this range considerably. A practical system calls for bit rates of the order of 5 bits a second or 50 watts of laser power with 10 joule pulses.
An alternate use of the laser energy would be to fire the laser every xcfx84 sec to obtain a more narrowband acoustic wave train centered on xcfx84xe2x88x921. For instance, a ten cycle burst at the same laser power per pulse (10J) cited above would require 100 joules. The bandwidth would be 11,000 Hz. Thus, if coherent detection could be used, an extra 10 dB of transmission loss could be tolerated.
A particle beam generates acoustic energy by impacting a small region of the surface of the water at the air/water interface. Energy from the aforementioned beam is absorbed by the water which causes the water to be heated. The heating of the water causes thermal expansion which generates pressure or stresses within the water that propagate through the water as a sound wave. The pressure P produced by the particle beam is given by expressions provided above for the thermoelastic energy case with the power flow in the particle beam replacing the laser power in the formulas.
Thus, by turning the particle beam on and off, a code similar to the one hereinbefore described may be produced because different amounts of energy will be absorbed by the water at different intervals of time causing acoustic signals to be produced which may be received by a sound detector.
The aforementioned beam is omni-directional and the apparatus of this invention produces directional signals by firing the abovementioned laser or particle beam at different locations on the surface of the water at predetermined times. Five beams are normally fired at the water in a given cycle to produce directional thermoacoustic signals. Each of the above beams is a pulse which propagates through the water as a series of disturbances, which look like expanding circles with most of their illumination in the forward direction of wave travel. In order for the beams to constructively interfere with each other in one direction and destructively interfere with each other in the other direction, the beams must be spaced one-half the wave length of sound apart and timed in such a manner that the next pulse is one period of the sound wave later. Thus, the foregoing concentrates the sound in one direction. Hence, if a 100 KHz signal is used, there will be 1000 microseconds between pulses that hit the water. The generation of the above pulses will not be exactly a thousand microseconds apart ("Ugr"o), since the speed of the vehicle that the pulse generation equipment is contained in and the angle that the pulses are directed at the water will affect the time between generation of the pulses in accordance with the following equation:
xcex94"Ugr"="Ugr"oxc2x7(1xe2x88x92{right arrow over (v)}{circumflex over (r)}/Co)
where {right arrow over (v)}=platform velocity, Co=speed of sound in water, and {circumflex over (r)} is a unit vector along the laser (or particle) beam direction.
Only a limited number of signal frequencies will add constructively and destructively in the direction you want the wave to travel. Each frequency travels in a certain direction.
Thus, acoustic waves will travel in a predetermined direction when a 100 KHz frequency is used. The acoustic frequency determines the wave length and spacing between pulses, i.e., frequency equals the speed of sound/xcex (the speed of sound is a known quantity that varies with temperature).
By producing a directional beam, the intensity of the signal may be reduced without decreasing the ability to receive the signal. Thus, when the foregoing is used in a sonar application, it is easier to locate the target and when the foregoing is used in a communications application, it is easier to illuminate the area near the receiver.
It is an object of this invention to provide a new and improved Steerable Thermoacoustic Array.
Other objects and advantages of this invention will become more apparent as the following description proceeds, which description should be considered together with the accompanying drawings.